Mr. Comey said that it was unusual for the F.B.I. to confirm or deny the existence of any investigations, but that in unusual circumstances when it is in the public interest, the bureau will sometimes discuss such matters.

“The F.B.I., as part of our counterintelligence effort, is investigating the Russian government’s efforts to interfere in the 2016 president election,” he continued, adding that the investigation included looking at whether associates of Mr. Trump were in contact with Russian officials, and whether they colluded with them.

Admiral Rogers made it clear that Russian efforts to interfere in democratic elections were not a one-off intrusion. They continue — now in Europe.

“We’ve seen some of the same things we’ve seen in the U.S. in terms of disinformation, fakes news,” he said.

President Trump, offering commentary on Twitter, put the best spin on it, noting that the witnesses did not say Russian hackers had changed vote tallies.

But Democrats clearly had the advantage, sending out campaign fund-raising appeals as the hearing unspooled.

Senator Chuck Schumer of New York, the Democratic leader, responded: “The possibility of coordination between the Trump campaign and Russian officials is a serious, serious matter. The investigation must be fair, independent, and impartial in every way, and the F.B.I. must be allowed to follow the facts wherever they may lead.”

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F.B.I. Director Confirms Russia Investigation

James B. Comey said that the agency is investigating Russian interference in the 2016 presidential election. Regarding President Trump’s tweets about alleged wiretapping by the Obama administration, Mr. Comey said the agency had “no information that supports those tweets.”

Mr. Comey told lawmakers that the investigation began in July, but he conceded that he had only “recently” briefed congressional leaders on the existence of the F.B.I. investigation. Asked why he had waited so long, he said, “Because of the sensitivity of the matter.”

Much of the questioning focused on the Trump confidante Roger J. Stone Jr. and his public bragging that he was in contact with the hacker Guccifer 2.0, believed to be a front for Russian intelligence officials. Mr. Comey would not confirm or deny the subject of the F.B.I.’s investigation, but Mr. Stone, never a shrinking violet, did speak up on Twitter.

It’s only fair that I have a chance to respond 2 any smears or half truths about alleged “Collusion with Russians” from 2day’s Intel Hearing

Mr. Comey told the House Intelligence Committee, “We have no information to support” President Trump’s assertion on Twitter that President Barack Obama wiretapped Trump Tower.

“We have no information to support those tweets,” Mr. Comey said, repeating moments later, “All I can tell you is that we have no information that supports them.”

The N.S.A. chief, Admiral Rogers, weighed in as well, saying that he had no knowledge of anyone asking the British or any other ally to wiretap Mr. Trump. That seemed to refute another claim made by the White House.

“I’ve seen nothing on the N.S.A. side that we engaged in such activity, nor that anyone engaged in such activity,” Admiral Rogers said.

Trump and Comey clash over F.B.I. conclusions.

Mr. Trump, watching the House Intelligence Committee closely, has offered a stream of comments on Twitter, attacking classified leaks and concluding that, in fact, Russia did not influence the electoral process.

Representative Jim Himes, Democrat of Connecticut, read those tweets to Mr. Comey, who sought to set the record straight.

“We’ve offered no opinion, have no view, have no information on potential impact because it’s not something we looked at,” Mr. Comey said, clarifying that the intelligence community was examining what Russia did to interfere with the election, not the impact of that interference.

Mr. Comey added: “It certainly wasn’t our intention to say that today because we have no information on that subject.”

Comey’s revelation raises questions about the Clinton email inquiry.

Mr. Comey’s statement that the F.B.I. is investigating the Trump campaign is certain to raise comparisons to his disclosure in October that the bureau had discovered a new trove of Mrs. Clinton’s emails.

“Some folks may want to make comparisons to past instances where the Department of Justice and the F.B.I. have spoken about the details of some investigations,” Mr. Comey said. “But please keep in mind that those involved the details of completed investigations. Our ability to share details with Congress and the American people is limited when those investigations are still open, which I hope makes sense. We need to protect people’s privacy. We need to make sure we don’t give other people clues as to where we are going.”

Mr. Comey said that he had consulted the Justice Department about whether to disclose the existence of the investigation — something he had not done before he held a news conference in July to announce that there was not enough evidence to charge Mrs. Clinton with a crime.

Mr. Comey’s statements on Monday brought immediate criticism from Mrs. Clinton’s allies and former campaign officials. “In refusing to discuss an ongoing investigation, Director Comey is appropriately adhering to the Justice Department’s standards,” said Brian Fallon, the spokesman for Mrs. Clinton’s campaign. “The question he has never satisfactorily answered is why he deviated from those standards so egregiously in Hillary Clinton’s case.”

Daniel C. Richman, a longtime confidant of Mr. Comey’s and a professor at Columbia University, defended Mr. Comey. “There is no fair comparison between announcing a material status change in an investigation that you publicly declared to be closed — in an announcement the public can be expected to rely on — and speaking about investigative moves, or nonmoves, in a covert national security investigation,” Mr. Richman said.

McCarthyism? Um, no.

Mr. Comey provided a rare moment of levity when asked if he believed the inquiry into Russian meddling and possible connections to the Trump campaign was a form of “McCarthyism.”

The question came during a stretch of questioning in which both he and Admiral Rogers unequivocally rejected claims by Mr. Trump that he was wiretapped during the campaign.

As for McCarthyism, Mr. Comey’s reply was similarly definitive: “I try very hard not to engage in any ‘isms’ of any kind, including McCarthyism,” he said.

But unsubstantiated charges were leveled.

Representative Trey Gowdy, Republican of South Carolina, who led the inquiry into Mrs. Clinton after the 2012 Benghazi attacks, seems to have turned into the Inspector Javert of intelligence leaks, with the president of the United States at his side. And who are they blaming? Much of the Obama administration — including former President Barack Obama himself.

Mr. Gowdy, in a question to Mr. Comey, asked: “Unauthorized dissemination is punishable by felony up to 10 years in federal prison?”

“Yes, as it should be,” Mr. Comey said.

Mr. Gowdy, who led the House Select Committee on Benghazi and helped expose Mrs. Clinton’s use of a private email server, asked if a series of people from the Obama administration had access to the intelligence information leaked: John O. Brennan, the former director of the C.I.A.; James R. Clapper Jr., the former director of national intelligence; Loretta E. Lynch, the former attorney general; Susan E. Rice, the former national security adviser; Ben Rhodes, a former National Security Council official; and … Mr. Obama.

He also listed a series of news reports from The New York Times and The Washington Post that detailed information gleaned from classified intercepts of calls between the Trump adviser Michael T. Flynn and Sergey I. Kislyak, the Russian ambassador.

Mr. Gowdy concluded by asking: “I thought it was against the law to disseminate classified information. Is it?”

“Yes, for sure,” Mr. Comey said. “It is a serious crime.”

The president himself chimed in on the issue via Twitter, again hinting that the leaks came straight from his predecessor:

FBI Director Comey refuses to deny he briefed President Obama on calls made by Michael Flynn to Russia. pic.twitter.com/cUZ5KgBSYP

Nunes: ‘There was not a physical wiretap of Trump Tower.’

The first hearing of the House Intelligence Committee’s investigation into Russia’s meddling in the election opened with Representative Devin Nunes of California, the committee’s chairman and a Trump ally, trying to split the difference between his hawkish view of Russia and his desire to deflect accusations that Mr. Trump’s campaign benefited from Russian interference in the election — or, worse, possibly colluded with Moscow.

Shortly after the witnesses — Mr. Comey and Admiral Rogers — took their seats, Mr. Nunes opened by stating that Russia had a long track record of aggressive actions against its neighbors, and that “its hostile acts take many forms aside from direct military assaults.”

Russia “has a long history of meddling in other countries’ election systems and launching cyber-attacks on a wide range of countries,” he said. “The fact that Russia hacked U.S. election-related databases comes as no shock to this committee.”

But in a nod to a claim pushed by Mr. Trump that he was wiretapped, Mr. Nunes said he wanted to know if there was improper surveillance of campaign officials. And he said it was important to find out “who has leaked classified information.”

“Numerous current and former officials have leaked purportedly classified information in connection to these questions,” Mr. Nunes said. “We aim to determine who has leaked or facilitated leaks of classified information so that these individuals can be brought to justice.”

House Intel Chair Nunes: "We know there was not a physical wiretap of Trump Tower" but it's possible other surveillance methods were used. pic.twitter.com/cSUBNxZqQB

Schiff: Collusion with Russia would be a ‘shocking’ betrayal of democracy.

Representative Adam B. Schiff of California, the committee’s ranking Democrat, laid out the case that the Trump campaign was, at best, far too close to Russia — and far too eager to appease Moscow on Ukraine and other issues.

“If the Trump campaign, or anybody associated with it, aided or abetted the Russians, it would not only be a serious crime, it would also represent one of the most shocking betrayals of our democracy in history,” he said.

Mr. Schiff said the committee also planned to examine whether the raw intelligence substantiated intelligence officials’ assessment and whether the government had reacted quickly and appropriately to revelations of Russia’s efforts to influence the election.

It is “unknowable” whether Russian meddling altered the outcome of the election, Mr. Schiff said, emphasizing that it mattered more that Moscow succeeded in intervening and would do it again.

“If we do not do our very best to understand how the Russians accomplished this unprecedented attack on our democracy and what we need to do to protect ourselves in the future,” he said, “we will have only ourselves to blame.”

The president doth protest.

With the House Intelligence Committee convening for its first public hearing on Russian election meddling, the president is posting his protests on Twitter — again.

James Clapper and others stated that there is no evidence Potus colluded with Russia. This story is FAKE NEWS and everyone knows it!

But Mr. Schiff asserted on Sunday that there was circumstantial evidence of collusion between Russian intelligence and Trump associates during the campaign. So protest as he might, Mr. Trump is not going to head off that line of questioning.